Multiply neutralized ion source



May 4, 1965 D. GABoR 3,132,220

' MULTIPLY REUTRALIZED ION SOURCE Filed March 18. 1960 s Sheets- Sheet 1DEN/V15 GABOR B/y yon-0w, Mm. T

Inveni'm' May 4, 1965 Filed Marh 18. 1960 D. GABOR 3,182,220

MULTIPLY NEUTRALIZED ION SOURCE 5 Sheets-Sheet 2 DEA/N16 6? 0R 3), KW W?or Cys May 4, 1965 p. GABOR MULTIPLY NEUTRALIZED ION SOURCE 5Sheets-Sheet 3 Filed March 18, 19 60 Ins/e 76:- DE NNIS C ABOR 7 I rncysy 4, 1965 D. GABOR 3,182,220

MULTIPLY NEUTRALIZED ION SOURCE Filed March 18. 1960 5 Sheets-Sheet 4 Inven'far QfN/VIS .519 R y 1965 D GABOR 3,182,220

MULTIPLY NEUTRALIZED ION SOURCE Fil ed March 18. 1960 5 Sheets-Sheet 5I'M/8015! 'DENNIS 6950K a; flaw, 422%.... 7

0"? eyS United States Patent 19 Claims cl. 313-63) This inventionrelates to large power ion beam devices. An object of the invention isto provide a device capable of functioning as a source of an ion beam ofhigh energy.

A further object of the invention is to provide a device capable ofdelivering a high energy ion beam in which space charge is neutralizedby electrons.

It is a still further object of the invention to provide a device inwhich a high energy ion beam may be set up and converted into an ionicflywheel.

More particularly the invention provides an ion beam source of a novelkind in which a large ion current which may be of the order of hundredsor even thousands of amperes is produced and accelerated to energies ofthe order of 5-100 kev. or more. A further optional element of theinvention consists in means associated with the said ion beam source,which shapes the ion beam into the form of a hollow cylinder, in whichthe ion rotate at high energies around in axis, while their space chargeis neutralized by electrons injected into the beam. This cylinder willfor brevity be referred to as an ionic flywheel, as it contains, like aflywheel, a large kinetic energy in the form of rotational motion.

According to this invention in one aspect an ion beam source comprisesmeans for setting up an ion stream and means for injecting electronstransversely into said stream at a plurality of locations along saidstream.

In another aspect the invention provides an ion beam source comprisingmeans for introducing gas molecules into an ionising location, means foragitating electrons released in said ionising location to ionise saidgas molecules, means for accelerating the ions so formed away from saidionising location and means for injecting further electrons transverselyinto the accelerating ion steam so as to neutralise the space change inthe accelerating region.

According to a feature of the invention the ion beam source may beassociated with means for diverting the ion beam into an ionic flywheel,the means for effecting this diversion comprising means for setting up amagnetic field in the form of a resultant of three componet fieldsnamely, a first azimuthal component, a second component derived from atleast one current loop surrounding the axis of symmetry of the ion beamsource and a third uniform axial component produced by at least one coilsurrounding said current loop and energized in opposite sense to saidloop.

The ion beam sources described hereinafter comprise two principal parts,an ion source proper, that is to say a location in which gas moleculesare ionized, and an accelerating region in which ion from the source areaccelerated to a higher energy level and formed into a beam. A thirdregion may be included in which the ion beam is converted into a ionicflywheel. In the ion source a high frequency field is used to agitatethe electrons and so produce ionization by collisions with the gasmolecules and fields are set up to cause the ions so generated to movetowards the accelerating region while the electrons are caused to movetowards the walls of the ionizing location where they are drawn off bysuitably placed electrodes. In the accelerating region an acceleratingfield is set up to provide acceleration of the ions away from the sourceand the space charge in this region is neutralized by means of electronsinjected transversely into the beam "ice at various points along it,acceleration of the electrons out of the beam being avoided by means ofan arrangement of electric and magnetic fields.

Ion beam devices according to the invention have rotational symmetryaround an axis and for clarity the various directions which are to bereferred to are defined as follows. Directions parallel to the axis willbe referred to as axial, directions at right angles to this in planescontaining the axis as radial, directions at right angles to V the axialand radial directions as azimuthal. Planes containing the axis will bereferred to as meridian planes, and directions other than axial orradial contained in these planes will be referred to as meridionaldirections. The general arrangement comprises means nearest the axis ofsymmetry for introducing gas molecules into an annular ionisationlocation or source and surrounding this is the acceleration region,which is also annular. The ion beam is accelerated radially outwardlyand given at the same time a rotational motion so that it leaves thisregion substantially tangentially and is then diverted into an axialdirection to form a cylindrical circulating ionic flywheel. A gas suchas hydrogen or deuterium is admitted, continuously or in short bursts,to the ion source. A strong high frequency azimuthal electromagneticfield is set up in this space, which will accelerate electrons to suchmean energies, preferably of the order 50-200 ev., that they not onlyionise the gas, but break up. at least part of the molecules, convertingat least a part of the neutral gas into protons or deuterons before thegas has pene trated beyond the ion source. The motion of the electronsis an energetic oscillation in the azimuthal direction with thefrequency of the HR field, with a superposed slow axial drift towardsone or both of the walls. The motion of the ions is mainly radial, thatis to say parallel to the walls of the ion source. This type ofoperation is made possible by the provision of a strong azimuthalmagnetic field which may be produced by a very strong current in theconductor located in the central axis. This strong azimuthal field slowsdown the sideways drift of the electrons to such an extent that themid-plane of the plasma between the two walls assume a potentialnegative in relation to that of the walls, so that a field is producedwhich tends to drive the electrons to the walls, while keeping the ionsaway from them. In the ideal operation of the device the drift of theions is radial, with a certain amount of rotation around the axis, butwithout axial components. In order to achieve this kind of operation,which produces maximum ion economy, electric fields are set up in theion source, which have a radial component which accelerates the ionsoutwards, and an axial component which just compensates the magneticforce produced by the radial motion of the ions in the azimuthal mageticfield. This field may be set up by electrodes located on both walls ofthe ion source, and arranged so as not to prevent the penetration ofhigh frequency electrical energy into it. They are therefore not annularelectrodes, but a plurality of point-electrodes. They may be connectedto the accelerating potentials, which are substantially steady duringoperation, by means of transmission lines which are a quarterwave-length long at the high frequency so that they appear assubstantially an open circuit for the high frequent energy in theionisation location, while presenting low impedances for the steadycurrents, that is to say for the electron currents which flow to themfrom the plasma.

Preferably the ion source is arranged in a region in which themeridional magnetic field is very small. This has the advantage that allions are produced in annular zones which are linked with very nearly thesame magnetic flux. Consequently, by the law of conservation of totalmomentum, all ions, wherever they have originated in the source, willaquire the same axial momentum in the magnetic fields to which they aresubjected later. In order to make the beam homogeneous as regards totalenergy also, the accelerating voltage drop in the ion source is kept aslow as possible, preferably within a few hundred volts. The result isthat all ions will have nearly the same dynamic data, as if they hadoriginated at one point.

The ions produced in the ion source now proceed into the acceleratingspace, which can also be called the ion gun. While in the ion source thespace charge of the ions is neutralized by the secondary electrons,which are produced in equal numbers. In conventional ion sources theacceleration is effected in vacuo, with a very strong ion space chargedeveloping, which reduces the avaliable ion currents to small fractionsof an ampere. In the present invention neutralization is affected byemploying for the acceleration electrodes capable of emitting electrons.In known designs of ion sources this would lead to a breakdown of thehigh accelerating voltage by arching bet-ween the electrodes. This isavoided in the accelerator according to the invention by twoco-operating means. One is a meridian magnetic field, which steadilyincreases in strength from the ion source outwardly attaining largevalue of the order of several thousand gauss in the outer zones of theaccelerator, where the accelerating electric gradient is large. Theother is the arrangement of annular accelerating electrodes in pairs inthe opposite walls of the accelerating space, in such a way that theelectrical equipotential lines joining them coincide with the magneticfield lines, that is to say the electrodes of one pair connected by amagnetic field line have the same potential. In a strong magnetic fieldelectrons can move easily only along the magnetic field lines, and asthese coincide with the equipotential lines, electrons will never begreatly accelerated, but will oscillate in the ion cloud, accumulatingin it to the extent which is necessary to compensate its space charge.The emission of electrons can be effected by hot cathodes, but also bycold metal electrodes, using the phenomenon of the unipolar arc. Thiseffect, which has constituted a frustrating phenomenon in manythermonuclear devices, consists in the formation of an arc dischargewhenever a plasma approaches a metal electrode whose potential isnegative with respect to the plasma. The result is that the plasma willbe charged up by electrons emitted by the electrode to within a score ofvolts of its potential. This phenomenon is used here to good purpose,because in the strong magnetic field the unipolar arc will charge up toa constant potential only one narrow zone of the ion beam, withoutdanger of destroying the gradient between this zone and the next, owingto the fact that in the strong meridian field the plasma is a badconductor at right angles to the said field, in the radial direction.

The high frequency electric field does not penetrate into theaccelerating space, as it is screened off by the annular electrodes, butthe azimuthal magnetic field extends into it. As this would bend the ionbeam, in order to maintain the ion motion in the required plane theequipotential lines are skewed relative to the direction of ion motion,to such an extent as to compensate the magnetic force component at rightangles to the said motion.

The invention as so far described, that is to say the ion beam sourceitself, has practical applications such as the production of neutrons instrong bursts, or the production of tritium by deuteron bombardment oflithium. Further extensions will now be described which are ofimportance in the application of the invention of the production of veryhigh temperatures and of thermonuclear power.

The ions leave the ion beam source with a radial and an azimuthcomponent of velocity, the azimuth i.e. rotational velocity beingpreferably large compared with the radial. In order to store up largekinetic energies the ions are now directed, by a combination of electricand magnetic fields into a hollow cylindrical space, preferably having asmall radial thickness compared with the radius and length of the saidspace, and they are trapped in this space, so that a high ion density isbuilt up in it. The space charge of the ions is neutralized by an equalnumber of electrons, injected by special hot or cold cathodes. Thisneutral assembly of rotating ions and electrons will be for brevitycalled a flywheel.

In the flywheel the ions rotate around the axis, and are alternatelyreflected at the inner and outer boundaries. At these boundaries the ionmotion is either purely azimuthal, or at most has a small axialcomponent. The major part of the flywheel is situated in a substantiallyuniform, axial magnetic field. The sign of this field is opposed to thatproduced by a circular loop or loops situated near to the ion source atthe axis. In other words, the magnetic flux through a circle drawncoaXially through the ion starts with a, say, negative value, andincreases steadily as the ion moves outwards, attaining a positive valueat or before the exit of the ion gun. By the law of constant totalangular momentum, which is the sum of the mechanical angular momentum ofthe ion and of the magnetic flux traversed by the ion, the angularmomentum of the ion will steadily and steeply increase in absolute valueas the ion circles outwards. As it reaches a certain radius, all itskinetic energy imparted by the accelerating voltage drop will be used upas rotational energy. Hence this is a maximum radius, at which the ionmust turn back. This determines the outer boundary of the flywheel. Butthere will also be an inner boundary, because an ion launched under theconditions as described can never reach the axis in the uniform field.At the axis the magnetic flux is zero, but as the ion has started at aradius with a negative flux inside it, the flux traversed by it is stillpositive, hence at the axis it would still have to possess a finitepositive angular momentum which is impossible. Consequently, if theconditions as previously specified are observed, the flywheel will belimited inside and outside by two radii, which can be brought closetogether by suitable design. Limitation of the flywheel in the axialdirection may be achieved by the simple device of increasing themagnetic field intensity at the required height, for instance by addingat this level an extra coil to the coil system which produces theuniform axial magnetic field.

Only very small energies could be stored in a flywheel without spacecharge neutralization because of the enormous space charge in ion beams.Space charge neutralization is achieved in the flywheel by electroninjection from a system of hot or cold cathodes arranged in coaxialcircles. In order to avoid ion losses at these electrodes, they are sopositioned that owing to the magnetic limitation of the boundary of theflywheel explained above, the ions pass close to them, but do not makecontact with them. When excess positive space charge starts accumulatingin the flywheel, the repulsion of the ions will make them approach thecathodes, until a neutralizing stream of electrons flows into it fromthe cathode, by hot emission or by a unipolar are. It is known that thisis possible only at the expense of losing some ions to the cathode, butthe ratio of ion current to electron current is about equal to thesquare root of the ratio of electron mass to ion mass. In the case ofdeuterons for instance this means that one ion is lost to the cathodefor about 60 electrons which flow into the flywheel, hence the loss isnot heavy.

The electrons injected into the flywheel will move freely along themagnetic field lines only, hence surfaces of revolution drawing throughthe field lines will automatically become equipotentials, withpotentials practically equal to that of the cathode which injected theelectrons. By impressing suitable potentials on the array of annularcathodes it is thus possible to regulate the electric potentialdistribution in the flywheel. This is an essential advantage of theinvention. It is well known that, even in discharge devices with exactlyrotationally symmetrical electrodes, systems of ions and/ or electronshave a tendency to depart from the rotationally symmetrical shape, andbreak up into uncontrollable strands. This danger is greatly reduced indevices according to this invention by the local control of potentials,which are made forcibly rotationally symmetrical.

Various equilibrium configurations of the flywheel can be produced inimpressing suitable potentials on the said injection-cathodes. Anequipotential flywheel is suitable for only small accumulations ofcharges. In such a flywheel the ions would describe very nearly circles,alternately contacting the inner and outer boundary. In a preferred formof the invention a radial electric field is impressed on the flywheel byan anray of cathodes, of such intensity that the ion trajectorie in thebulk of the flywheel become straight lines, which are fairly abruptlyreflected at the outer boundary. In this preferred operation the ionsand the electrons rotate. together in the bulk of the flywheel, theelectrons on coaxial circles, the ions along polygonal paths. Hence inthis operation not only the space charge but also the ion current isneutralized in the bulk of the flywheel. This has the advantage that theposition of the flywheel remains unchanged during the build-up period,while the large equal positive and negative charges accumulate in it,hence the injection conditions remain constant. But it is not possibleto maintain current-neutrality in the whole of the flywheel. The ionsmust be reflected at the outer boundary 'by a rather abruptly increasingmagnetic field, which is screened off by a circulating current. In thepreferred operation according to the invention this current isconcentrated in a rather thin surface layer at the outside of theflywheel. It can be seen that this current must have the sign of an ioncurrent, but it is preferable to produce it not by ions but mainly byelectrons circulating opposite to the ions in the said outer surfacelayer or skin. Consequently the radial electric field, which in the bulkof the flywheel was of such sign as to accelerate ions outwards, must bereversed in the said skin.

The magnetic field at the outside of the flywheel must be stronger thanthe field at the inner boundary, because the magnetic pressuredifference must be such as to balance the centrifugal force in therotating flywheel. Consequently the magnetic field at the outside mustbe increased as mass and charge accumulate in the flywheel. This can beachieved by allowing the flywheel to expand a little and to compress themagnetic field between it and the conducting vacuum envelope. It is,however, preferable to keep the flywheel in its original position duringthe build-up process, and to increase the magnetic field produced by theouter coils, by increasing their current according to a certainschedule.

In a preferred operation of the invention the flywheel is built uprapidly, for instance in one millisecond, until it contains e.g. ionecoulomb charge. This requires an ion current of 1000 amperes. With anaccelerating voltage of e.g. 100 kilovolts the energy stored in theflywheel, almost entirely in the form of kinetic energy of the ions,will be 100,000 joules. Rapid build-up has the advantage that the ionmotion remains essentially regular for short times. In longer times theions will collide with one another and develop a random radial motionsuperimposed on their regular rotation. In even longer times the ionswill share their random energy with the electrons, and these willradiate it away. Once the ions have shared their energy with theelectrons the flywheel changes into a rotating plasma, with a certaintemperature, and it is known that plasmas will diffuse through magneticbarriers. Hence precise confinement of the flywheel is facilitated byrapid build-up. On the other hand a loss of a small percentage of theenergy of the ions to the electrons is beneficial, because ions whichhave lost energy in the flywheel will not be able to retrace their stepsand flood back to the ion gun. The time 6 of build-up is therefore amatter of advantageous compromise between the beneficial and deleteriouseffects.

A flywheel containing e.g. 1 coulomb of charge of either sign, i.e. 6.10ions in a volume of, for instance 10 cm. with an energy of e.g. kev.offers an advantageous starting point for the production ofthermonuclear power. One way of achieving this is to start with aflywheel of deuterons and to admit to the vacuum envelope containing theflywheel an equal molecular quantity of tritium gas, that is to say onetritium atom for two deuterons. After the collision of the rotatingdeuteron flywheel with the practically stationary tritium, the totalrotational energy will drop to about 40% of the original, and 60% of theenergy becomes available for sharing between deuterons, tritons and anequal number of electrons. This is rapidly randomized, and is convertedinto a temperature of several hundred million degrees, suflicient forstarting the D-T fusion reaction. In order to make this economical it isnecessary, however, to compress the mixture about a hundred times byrapid increase of the magnetic fields. The flywheel will then beconverted into a very hot toroidal plasma in which the fusion reactionwill go on so long as the magnetic fields can contain it. The deviceaccording to the invention has an important advantage over the knownso-called mirror machines in that the plasma is torodial, and willremain so owing to its rotation which is not randomized. Consequentlythe plasma will not leak off through the axis, as in conventional mirrormachines. Moreover, it will carry a certain large current, and therebyprovide by itself a'certain fraction of the magnetic fields required forits containment. A further advantage is that an azimuthal stabilizingmagnetic field can be impressed on this toroidal plasma through thecentral conductor.

The invention will be better understood from the following descriptionof two embodiments thereof given with reference to the accompanyingdrawings in which:

FIG. 1 is a schematic illustration of the two magnetic fields whichtogether compose the meridonal magnetic field.

'FIG. 2 illustrates the result of the superposition of these two fields.

FIG. 3 is a section through a first form of the ion beam source, with aschematic illustration of the limitations of the flywheel.

FIG. 4 is a side view, partially broken away, of the whole device ofFIG. 3.

FIG. 5 is a horizontal sectional view of the device of FIG. 3 takensubstantially on the line 5-5 in the latter figure, but omitting certainof the elements in the interest of clarity.

FIGS. 6 and 7 are fragmentary sectional views of two alternative formsof the injection cathode of FIG. 3.

FIG. 8 is an axial section through an improved form of the ion beamsource, accelerator and reflector with cathodes for injecting electronsinto the flywheel.

FIG. 9 is a fragmentary horizontal sectional view of the device of FIG.8 taken substantially on the line 99 in the latter figure, but omittingcertain of the elements in the interest of clarity.

FIGS. 1 and 2 illustrate the design of the meridional magnetic field forthe ion beam source. 1 is a conductor forming a circular loop around theaxis Y. The magnetic field is represented not by field lines but by thelines of constant vector potential A, which is defined as A =/21rr,where r is the radial distance from the axis, and 4) the magnetic fluxpassing through the circle of radius r. The lines A=a constant aresomewhat different from the field lines which are the lines of constantflux, i.e., =a constant. They have the advantage over a representationby field lines that a uniform axial magnetic field is represented byequally spaced lines A=a constant, it A increases in equal steps. FIG. 1shows the two components of the meridional magnetic field separately.The A=a constant lines of the first component field, produced by thecurrent in the loop 1, form closed lines around it. The second componentfield is produced by coils outside the limits of FIGS. 1 and 2, butillustrated as coils 36 in FIGS. 4, 5 and 9. This second field isuniform near the level of 1, and for some distance above it, up to thelevel to which one wishes the flywheel to extend. The field intensityincreases immediately below the level of 1 and above the previouslymentioned top level.

In FIG. 2 the field resulting from the superposition of the twocomponent fields is shown. As the currents in the loop 1 and in theabove-mentioned outer coils 36 have opposite signs, the result is thatthey weaken one another inside the loop 1, and reinforce one anotheroutside it. At any point of a certain line A=0, shown in dashed lines,the flux between the point and the axis is zero. This line is thereforealso a field line, 1 5:0.

Elsewhere field lines and lines A=a constant differ from .one another,as shown by the field line F, shown in dotdashed lines passing throughthe point A=-3 at r=r In the present example it is assumed that the ionsoriginate at this point A r The figure shows also, in dashed lines,certain contours of constant rotational energy. These are determined,apart from a constant factor, by the formula m= o e 2 Any one of theselines is the boundary of the accessible space for ions which have beenaccelerated by the electric voltage drop to a total energy equal to thefigures marked on these lines, because at these lines the whole energyis used up in rotation. It is seen that all ions of given energy arethereby confined to closed regions.

FIG. 3 is a schematic axial section, FIG. 4 is a side view and FIG. 5 isa cross-section through one design of the ion beam source, FIG. 3 alsoillustrating the shaping of the flywheel by the magnetic fieldspreviously described. An axial tubular conductor 2 which carries theaxial current I runs through the whole structure and produces a strongazimuthal magnetic field. In this design a mechanically controlled gasinlet is provided. A movable circular plate 3 preferably of ceramicmaterial is arranged between two dishes 4 and 5, and can be raised bymeans of a solenoid 6. A diaphragm 7 of silicone rubber or the like fitssnugly to the bottom plate and closes the gas inlet 8. On raising 3 acertain small amount of gas is admitted through an annular gap betweenthe two dishes 4 and 5 into the ion source. In this drawing the circularloop 1 is placed immediately below the entrance 9 of the ion sourcespace 10, and this loop carries, in addition to the steady current whichgenerates a component of the meridional magnetic field, a high frequencycurrent for energizing the source, that is to say for ionizing the gasstream on its way through it. The source is schematically indicated asbounded by a double array of circular cathodes 11, which must not befull loops, or else the high frequency power could not penetrate intothe space between them. The ion source space 10, that is to say thespace in which ionization of the gas stream takes place, extends as faras two outer dishes, 12 and 13, preferably of ceramic material, whilethe accelerating space 14 is outside them. This space is flanked bycathodes 15, connected in opposite pairs, which carry voltages graduallydecreasing towards the outer periphery. As previously explained, inspite of the high potential differences between neighbouring electrodepairs 15, no discharge will take place between these, by reason of thestrong meridional magnetic field produced by the loop 1 and the outercoils which prevents electrons from moving radially.

FIG. 3 illustrates also how the ion beam is diverted into an ionicflywheel, in the simple example of an equipotential flywheel. The linesof constant rotational ion energy, shown in dashed lines, aretransferred into this drawing from FIG. 2. As an example, the numberingof these lines may correspond to kilo-electron-volts; for instance theline 56 means that this is the outer boundary of the region accessiblefor ions with 50 kev. energy. At the lower end of this region aninjection cathode 16 is arranged the contour of which follows this linealong a short are. Ions accelerated to 50 kev. will be just able toreach this surface. As the ions accumulate, the ion cloud takes on amore positive potential, and a discharge occurs between 16 and thecloud, injecting electrons into it. As previously explained, this can beeither thermal emission or a unipolar arc discharge produced by anysuitable form of emissive cathode. A known form of thermal emissivecathode is illustrated in FIG. 6 and comprises an electron surface ofcontoured shape provided with heater wires in thermal contact therewith.A known form of unipolar arc discharge cathode is illustrated in FIG. 7and comprises a row of sharp metallic teeth forming a contoured surface.The result is that the ion cloud becomes neutralised, but only in theregion which electrons can reach. This region crosshatched in thedrawing is limited by two of the dotdashed lines starting from thecathode, which represent field lines, and which, by reason of the largemobility of the electrons in the direction of magnetic field lines,

ecome equipotential lines. It may be noted that in the presence of astrong azimuthal field the field lines are not in meridian planes, butform helices around the axis, but this does not affect therepresentation in the drawing, because the generatrix of the surfacescontaining the field lines is still determined by the meridionalmagnetic field alone.

The flywheel will thus occupy the cross-hatched area, which has a longcylindrical extension in the region in which the magnetic field isuniform and axial, a region which has been indicated by a gap in thedrawing. At the top, the flywheel is limited by the increased magneticintensity at this level.

The vacuum boundary 17, which is preferably in the form of a metallicenvelope evacuated by a suitable pump through a pipe 37 (see FIG. 4), ispreferably maintained at a potential equal to that of the injectioncathode 16, ie 50 kv. below that of the ion source in the above example,or somewhat lower. Coils 36 outside vacuum boundary 17 providing amagnetic field which, as previously described with reference to FIGS. 1and 2, forms the second component of the meridional magnetic field.

An improved design of the ion beam source is shown in FIG. 8 in axialhalf section and in cross-section in FIG. 9. Starting from the axis, thecentral conductor 2 which carries the axial current I producing theazimuthal magnetic field is a tube, provided with perforations 18 at thelevel of the ion source for the admission of dosed quantities of gas.Dosing is achieved by injecting, by a syringe or the like, a certainsmall volume of gas at the bottom of a column of mercury 19 covered by alayer of silicone oil 20 supported in a tube 21 housed within the tube2. The gas will rise in the tube 21 in the form of a bubble. In the tube21 there is an insulating ring 22 which insulates the end portion 23from the main part of the tube 21. When a gas bubble breaks the columnof mercury at this region the end portion 23 is isolated and a signal isthus provided which is fed through lead 24. This signal is used to startthe electrical supplies.

The gas proceeds from the holes 18 in tube 2 outwards between twoceramic plates 4 and 5 as before into an annular space which has aconstriction at 9, preferably provided with tangential grooves, so as toslow down the speed with which gas molecules travel through the ionsource space 10.

The means for generating the meridional magnetic field differ from thoseshown in FIG. 3 in that the loop 1 has been split into three parts, 1',1", and 1". 1 has two turns, and all four turns in 1', 1" and 1 carrythe same current, in like sense, in series. This has the advantage thatthe centre of the ion source space 10 is now at a point of zeromeridional field, and the field density is very small throughout thisspace, thus ensuring uniformity of angular momentum of the ion beam, aspreviously explained. A further difference is that the high frequencycurrent which induces the HR field in the source and thereby effectsionization therein, has been separated from 1 and is now a separate coil25.

As explained in the introduction, the design of the ion source is suchthat the ions formed in the space 10 by HE. bombardment proceed radiallyoutwards, while the secondary electrons produced in the ionizationprocess proceed to one or both of the walls. In this design of the ionsource the meridional magnetic field is weak while the azimuthal field,produced by the central current I is strong. The effect of this is tobend the ion trajectories in the meridian plane. In FIG. 8 it is assumedthat the sign of this field is such as to bend the trajectoriesdownwards. In order to compensate this effect the electricequipotentials are skewed in such a way that there is an axial componentof the electric field which tends to bend the trajectories upwards, justcompensating the effect of the magnetic field. This skew system ofequipotentials is produced by two radially offset arrays of electrodes26 and 27 corresponding ones being at the same steady potential.

The electric supply to the ion source must be such that it offers asmall impedance to the strong electron current, equal to the ioncurrent, which flows out through the said electrodes, while offering ahigh impedance to oscillatory currents of the frequency of the H.F.power supply. This is achieved in the present design by making theseelectrodes 26 and 27 in the form of a double array of wires embedded ina suitable dielectric 28, 29 so that the length between the inner endsin contact with the ion source and the outer ends at which they areconnected together by annular metal bands 30 and 31 and therebyazimuthally short-circuited is equal to a quarter of the vacuumwavelength corresponding to the said high frequency. Convenientfrequencies for the operation of this device are in the range 10-30meters vacuum wavelength. By embedding these conductors in a certaintitanateceramic, known under the registered trade name of Faradex H,which at the said frequencies has a dielectric constant of about 2500,the quarter-wave wires embedded in the said material can be made to havelengths of only 15 cm. The embedding must be very perfect, as any smallair gap between the wires and the ceramic will substantially increasethe length of the quarter-wave lines; hence it is preferred to fire theceramic after the wires are embedded in a powder or paste of the saidmaterial.

By breaking up the electrodes which supply the ion source with steadypotentials into a double array of quarter-wave wires, connected intoannuli at the outer ends only, two purposes are achieved. One is thatthe high frequency power can penetrate almost freely between the wiresinto the ion source. The other is that the HR impedance of the saidelectrodes is very large, hence no H.F. oscillating current can developin the discharge space, because it cannot continue in the outer circuit.This is of particular value if one desires the ions in the said space tooscillate as well as the electrons, and to contribute to breaking up thegas molecules. In this case instead of a completely azimuthal magneticfield one operates the device with a slightly helical field, preferablywith an inclination angle of the order (m/M) m/M being the mass ratio ofelectrons to ions. This is about 1/60 for deuterons. The effect is thatthe ions too will oscillate, with an energy about equal to theelectrons, but because of their higher mass they will describe orbits ofmuch smaller dimensions. By making the HF. impedance of the outercircuit large by the means described the plasma will automatically keepat a certain small distance from the conductors, because no H.F. currentcan flow through these. Such a slightly helical field can be achieved bymaking the meridional magnetic field in the beam sources not exactlyzero, but giving it a slight axial component.

The steady potential supplies to the ion source through the metal bands30 and 31 have a low potential, and can be led through the vacuum, asshown at the top and bottom of the figure. On the other hand the highvoltage supplies to the hot or cold cathodes 15 in the accelerating part14 of the ion beam source must be well insulated throughout, by ceramicconduits 32 and 33.

FIG. 8 differs from FIG. 3 also in the way in which the flywheel iscontrolled. While in FIG. 3 the flywheel branches out upwards anddownwards, in FIG. 8 it has only a single upward arm. This is producedby a reflector 34, preferably of ceramic material, in which is embeddedan array of annular injection cathodes 35. By giving these cathodesappropriate potentials, only a little less than the maximum potentialdrop, the ion beams leaving the ion beam source can be deflected bydegrees. The potential gradient between these cathodes regulates theradial potential distribution in the flywheel. In a preferred operation,as previously explained, the electric gradient is directed outwards andhas such a value that it just compensates the inwardly directed magneticforce, so that inside the bulk of the flywheel, the ions move instraight lines. They are reflected in a thin outer layer or skin of theflywheel, which alone carries a nonzero circulating current. In thislayer the electric gradient is reversed. This gradient too can beimpressed by the cathodes 35, but the said layer can also be left toitself. By the high conductance of the plasma, due chiefly to electrons,a current will automatically develop in the outer layer of suchintensity as to screen off the outer, strong magnetic field because, bythe means described, the circulating currrent is suppressed in the innerlayers of the flywheel. As previously described, in order to keep theflywheel in a fixed position during the build-up period, the outermagnetic field must be run up during this time from a small Value to alarger one, sufficient to compensate by its magnetic pressure thecentrifugal pressure of the rotating ions.

The means for the electrical operation of the ion beam source asdescribed are well known in the art of electrical engineering and needonly be briefly listed. The large central current I, which may be of theorder of e.g. 200 kiloamperes, can be advantageously produced by ahalf-wave of mains frequency, 50 or 60 cycles/sec, in a step-downtransformer, with synchronous switching at the zero points of thecurrent on the primary side. The same technique is also advantageous forthe currents circulating in the loops 1', etc. which are of a somewhatsmaller order, and for the coils outside of the tube which produce theuniform axial field. It is preferable to have the operating period nearthe maximum of the said currents, at which moment they are almoststeady. In order to ensure a safe balance of the fields at the positionof the ion source it is advantageous to have these loops and coils allin series. These and all other supplies are preferably timed by thearrival of the gas bubble at or near the top of the liquid column, butthe start of this bubble must be timed in relation to the mains, so thatthe bubble arrives near the time of a current maximum. At the momentwhen the gas arrives in or near the top of the tube 21, the highfrequency power supply is switched on. The steady potential supplies tothe ion source and accelerator and to the injection cathodes can bepreviously energized, because these will discharge no current until theion beam has started.

Instead of building up the rotating ion mass into a high energy flywheelthe invention may also be used for producing an axial ion beam. This maybe done by locating the ion source at any radius on the surface markedA=0 in the diagram of FIG. 2, that is to say on the surface whichencloses zero magnetic flux. In addition the axial magnetic field whichis uniform in the region of the source is allowed to fall to zero at alevel above the source. This is in contrast with the previous flywheelcontaining field which is made stronger at the higher level as shown inFIG. 2. With such a field, and with source located as described, theions will start rotating around the axis but they will gradually bealigned with it as they enter the region of decreasing field and finallyleave in an exactly axial direction.

Such an arrangement may be used for ionic bombardment of targets such aslithium or could be employed, for example, for propulsion of a spacevehicle.

It will be appreciated that such a beam will still be neutralized alongits whole length by the electrons injected as previously described.

I claim:

1. Ion beam source comprising means for setting up an ion stream, meanspositioned at a plurality of locations along said stream for injectingelectrons transversely into said stream so as to neutralise the spacecharge of the ions in said stream, and means for preventing flow ofinjected electrons between difierent electron injecting means.

2. Ion beam source comprising means for introducing gas molecules intoan ionising location, means for agitating electrons released in saidionising location to ionise said gas molecules, means for acceleratingthe ions so formed away from said ionising location, means for injectingfurther electrons transversely into the accelerating ion stream so as toneutralise the space charge in the accelerating region, and means for soconstraining injected electrons as to prevent acceleration thereof bysaid accelerating means.

3. Ion beam source comprising a vacuum envelope, means within saidvacuum envelope defining an ionisation location, means for acceleratingions away from said ionising location, means for injecting electronstransversely into the stream of accelerating ions, said electroninjecting means comprising at least one array of electrodes in closeproximity to the stream of ions, and

means for providing easy flow paths for the injected electronstransverse to the ion stream.

4. Ion beam source comprising a vacuum envelope, means within saidvacuum envelope for setting up an ion beam, and means for acceleratingthe said beam comprising at least one array of electrodes insuificiently close proximity to said beam that electrons will be emittedtherefrom transversely to said beam, means for applying potentials tothe electrodes of said array in descending gradation to provide anaccelerating field, and means for setting up a magnetic field in adirection transverse to said beam and perpendicular to the electricfield set up by said electrode array.

5. Ion beam source comprising a vacuum envelope, means within saidvacuum envelope for setting up an ion beam, and means for acceleratingthe said beam comprising an array of electrodes on each side of the beamin sufiiciently close proximity thereto that electrons will be emittedtherefrom, means for applying potentials t9 the electrodes of each arrayin descending gradation with the electrodes of one array forming pairsof equal potential with the electrodes of the other array, and means forsetting up a magnetite field transverse to the beam and the field linesof which are such as to pass through both electrodes forming anequipotential pair.

6. Ion beam source comprising in a vacuum envelope means defining anionisation location rotationally symmetrical about an axis, means forintroducing gas molecules into said location, means for agitatingelectrons released in said location by ionisation of gas gas molecules,means for drawing of said electrons in an axial direction, and means forsetting up an azimuth magnetic field within said location.

7. Ion beam source comprising in a vacuum envelope means defining anannular ionisation location centered on an axis, means for setting upwithin said location a high frequency electromagnetic field, means forimpressing a radial electric gradient across said location, and meansfor setting up an azimuthal magnetic field within said location.

8. Ion beam source comprising in a vacuum envelope means defining anannular ionisation location centred on an axis, means for introducinggas molecules into said location, means for setting up in said locationa high frequency electromagnetic field, means for setting up across saidlocation a radial electric gradient, means for setting up within saidlocation an azimuthal magnetic field and, surrounding said location anannular accelerating egion, means for setting up a radial potentialgradient across said region, means for introducing electrons into saidregion, and means for setting up in said region a magnetic fieldtransverse to said beam and to the electric gradient in saidaccelerating region.

9. Ion beam source as claimed in claim 8 wherein said means for settingup within said ionisation location a radial electric gradient comprisesa plurality of concentric arrays of conductors extending away from theboundaries of said location on either side of the beam, the conductorsof each array being connected together at their outer ends, the lengthof said conductors being electrically equivalent to a quarter wavelengthat the frequency of the high frequency excitation in the ionisationlocation.

10. Ion beam source as claimed in claim 9 wherein said conductors areembedded in dielectric material.

11. Ion beam source as claimed in claim 8 wherein the means for settingup the azimuthal magnetic field in said ionisation location comprises aconductor passing axially through said vacuum envelope.

l2. Ion beam source as claimed in claim 11 wherein the axial conductoris in the form of a tube, and wherein gas is introduced to theionisation location through this tube.

13. Ion beam source as claimed in claim 8 wherein the means for settingup within said accelerating region a radial potential gradient comprisesa plurality of electrodes located on each side of the beam, each of saidelectrodes encircling the axis at a different radial distance withoutforming a completely closed loop, the electrodes on one side of the beambeing paired with the electrodes on the other side of the beam to formequipotential pairs defining equipotential surfaces skewed to the axialdirection.

14. Ion beam source as claimed in claim 13 wherein at least some of saidelectrodes are electron emissive.

l5. Ion beam source as claimed in claim 8 including means for divertingthe ion beam into an ionic flywheel, said means comprising means forsetting up a magnetic field in the form of the resultant of threecomponent fields, namely, an azimuthal component field, a meridionalcomponent field derived from at least one current loop surrounding saidaxis, and a uniform axial component field produced by at least one coilsurrounding said loop, and energised in opposite sense to the said loop.

16. Ion beam source as claimed in claim 15 wherein said meridionalcomponent field is produced by at least two current loops encircling theaxis.

17. Ion beam device as claimed in claim 15 including means fordeflecting the beam into the axial direction.

18. Ion beam device as claimed in claim 17 wherein said deflecting meanscomprises an array of annular electrodes surrounding said acceleratingregion, and means for applying potentials thereto.

19. Ion beam source comprising means for setting up an ion stream, meanspositioned at a plurality of locations along said stream for injectingelectrons transversely into said stream so as to neutralise the spacecharge of the ions in said stream, and means for setting up a magneticfield in a direction transverse to said stream so as 13 14 to cause theinjected electrons to move along the mag- 2,873,400 2/59 Cook 313-231netic field lines. 2,880,337 3/59 Langmuir et a1 313-2315 ReferencesCited by the Examiner 2,392,114 6/ 59 p 3113-53 UNITED STATES PATENTS 5GEORGE N. WESTBY, Primary Examiner.

2,576,601 11/51 Hays 250-4191 ARTHUR GAUSS, RALPH G. NILSON, Examiners.

1. ION BEAM SOURCE COMPRISING MEANS FOR SETTING UP AN ION STREAM, MEANSPOSITIONED AT A PLURALITY OF LOCATIONS ALONG SAID STREAM FOR INJECTINGELECTRONS TRANSVERSELY INTO SAID STREAM SO AS TO NEUTRALISE THE SPACECHARGE OF THE IONS IN SAID STREAM, AND MEANS FOR PREVENT-